This invention relates generally to the field of intermittent linear motors for use in combustion gas powered tools such as those used to drive fasteners.
The cycle of the intermittent linear motor is different to that of a continuous running engine. It does not continue automatically, as would be the case in a reciprocating internal combustion engine. Instead, the intermittent linear motor""s piston must be returned to, and remain in, an initial starting or rest position between each power stroke. Typically, a rod fitted to the piston engages a fastener or other load and mechanical energy is transmitted through the rod to drive a fastener or perform other useful work during the power stroke.
The piston is returned to its initial starting or rest position during a reciprocation stroke with a resilient member. This stroke is not generally used for compression purposes as in a conventional compression engine. Instead, the cylinder is vented during reciprocation so that the contents of the combustion chamber in the rest position are at or near atmospheric pressure. This is primarily done because holding a compressed charge for what may be extended periods between cycles has not proven practical. However, as a result of the inherent thermal-to-mechanical output inefficiencies resulting from this lack of compression, the combustion chambers of intermittent linear motors are required to be fairly large for a given power output.
These relatively large uncompressed combustion chambers of intermittent linear motors, as well as being inherently inefficient, are especially sensitive to the presence of residual exhaust gases from previous cycles. Failure to remove such residual gases will result in a diluted charge and deterioration of burn speed, which is critical when driving a fastener. Thus, unless such gases can be substantially completely removed and replaced with a clean air/fuel mixture, subsequent cycles will deliver significantly less power.
It is, therefore, necessary to provide some type of efficient exhaust scavenging system in devices utilizing intermittent linear motors. Such systems seek first to discharge exhaust gases from the tool as quickly as possible after combustion has been completed and useful work performed. This helps prevent the tool from overheating and also minimizes the amount of scavenging air required to completely clean out the remaining exhaust gases. There can be some variation due to the differing shapes and configurations of combustion chambers and their porting locations; however, it is generally necessary to pump clean air having a volume at least 2.5 times the volume of the combustion chamber in order to adequately clean out (i.e.xe2x80x94scavenge) exhaust gases prior to injecting fuel into the chamber. Representative prior art approaches to the problem of rapidly and efficiently scavenging exhaust gases can be seen in U.S. Pat. Nos. 4,403,722; 4,712,379; and 4,759,318.
These patents generally rely on a temperature drop in the gases remaining in the cylinder after exhaust gases have been allowed to escape following a power stroke. This temperature drop forms a partial vacuum, causing scavenging air to be drawn in through check valves at the ignition end of the combustion chamber. A critical problem associated with these systems is the speed with which the scavenging operations of this type can be accomplished. As it takes time and temperature drop for a vacuum to be realized after the fastener has been driven, hot gases are allowed to stay in the tool for long period of time up to 500 milliseconds. This causes the tool to heat up and lose power as well as severely limiting the operating speed of the tool.
In my current invention, a novel approach has been taken to address the problems described above, allowing rapid automatic operation in a simple device. Unlike my U.S. Pat. Nos. 4,712,379 and 4,403,722, which rely on a vacuum being set up and manual operations to complete their cycles, exhaust gases can be completely scavenged within 10 milliseconds in the cycle of my invention. This allows for very rapid cycling rates and minimal heating of the tool. It shares the advantages of my U.S. Pat. Nos. 4,759,318 and 4,665,868 as its cycle is initiated solely by electric signal without the need for manual pumps or valves, but does not require numerous complicated valves and seals. Thus, it represents a significant advance in efficiency and simplicity of operation over prior art devices.
The present invention relates to an improved combustion gas powered intermittent linear motor having a combustion chamber and, an associated piston reciprocating in a piston chamber; the piston powered in a power stroke by ignition of gas in the combustion chamber and biased to return to rest in a return stroke, when not powered by the ignition of gas. An exhaust valve is associated with the combustion chamber, which valve opens to exhaust spent combustion gases and air from the combustion chamber after combustion. A plenum chamber is provided, this plenum chamber being in fluid communication with the piston chamber below the piston remote from the combustion chamber. The plenum chamber is also in communication with the combustion chamber. The motor is configured so that:
(a) air is compressed in the piston chamber below the piston during the power stroke and this compressed air is compressed into the plenum chamber;
(b) then, as the combustion chamber pressure drops, the compressed air from the plenum chamber flows through the combustion chamber, and subsequently through the exhaust valve, scavenging the combustion chamber of spent combustion gases;
(c) as the plenum chamber pressure drops and the piston is on its return stroke, the piston draws in air from below it through an air inlet means in the piston chamber while exhaust gases above the piston are being forced out through the exhaust valve; and
(d) as the pressure in the combustion chamber and plenum chamber return to substantially atmospheric pressure, all valves close to ready the motor for fuel injection and ignition.